In an optical communication network, optical signals having a plurality of optical channels at individual wavelengths, called “wavelength channels”, are transmitted from one location to another, typically through a length of optical fiber. An optical cross-connect module allows switching of optical signals from one optical fiber to another. A wavelength-selective optical cross-connect, or wavelength selective switch (WSS) module, allows reconfigurable wavelength-dependent switching, that is, it allows certain wavelength channels to be switched from a first optical fiber to a second optical fiber while letting the other wavelength channels propagate in the first optical fiber, or it allows certain wavelength channels to be switched to a third optical fiber. An optical network architecture based on wavelength-selective optical switching, which is sometimes referred to as an “agile” optical network architecture, has many attractive features due to the ability to automatically create or re-route optical paths of individual wavelength channels. It accelerates service deployment, accelerates rerouting around points of failure of an optical network, and reduces capital and operating expenses for a service provider, as well as creating a future-proof topology of the network.
Conventional WSS modules have been constructed to switch wavelength channels between one input optical fiber and a few, for example four or eight, output optical fibers. For example, the folded symmetrical 4-f configuration disclosed in U.S. Pat. No. 6,498,872 by Bouevitch et al., and the optional field-flattening optical wedge taught in U.S. Pat. No. 6,760,501 by Iyer et al., both assigned to JDS Uniphase Corporation and incorporated herein by reference, allow construction of WSS modules for performing the abovementioned wavelength channel switching function. Multiport WSS modules are also taught in U.S. Pat. Nos. 6,707,959 by Ducellier et al. and 6,810,169 by Bouevitch, both assigned to JDS Uniphase Corporation and incorporated herein by reference, while a multi-module unit is taught in US Pat. Appl. Pub. No. 20070242953 by Keyworth et al., which is also incorporated herein by reference.
The abovementioned 1×N WSS modules, although useful in the agile optical networks as mentioned above, are limited by having only one input port (or only one output port when used in a reverse direction). One such limitation is related to having wavelength channels at the same wavelength in the same network. Since the wavelengths of all wavelength channels have to be different at any single port to avoid undesired interference, having one input or one output port in a WSS device results in the entire device being incapable of handling more than one “instance” of a wavelength channel. Another limitation is related to reliability and redundancy requirements. Having all the traffic propagating in a single optical fiber connected to the single input or output port of a 1×N WSS lowers the reliability of an optical network, because damage to that single fiber may result in a catastrophic failure of the entire network. Accordingly, there is increasing interest in M×N WSS modules for use in agile optical networks.
Traditionally, M×N WSS were provided by connecting M×1 and 1×N WSS modules, either in series or in parallel. Referring to FIG. 1A, a compound M×N WSS module 100A is shown having a M×1 WSS module 101 and 1×N WSS module 102. The modules 101 and 102 are connected serially with a common optical fiber 103. The combined module 100A has M input ports 104 and N output ports 105. Unfortunately, the WSS 100A is “wavelength-blocking”, meaning that it does not allow routing of wavelength channels at the same wavelength, appearing at the different input ports 104. Referring to FIG. 1B, a compound N×N WSS module 100B is shown having 2N 1×N WSS modules 106 interconnected with N fiber bundles 107. The WSS module 100B is “non-blocking”, however this is achieved at a very high cost of having to use many 1×N WSS modules 106. Furthermore, both modules 100A and 100B have high insertion loss, since an optical signal has to pass through two modules.
U.S. Pat. No. 6,711,316 by Ducellier, assigned to JDS Uniphase Corporation and incorporated herein by reference, discloses a N×N wavelength cross-connect having two N×K arrays of beam deflectors, wherein K is the number of wavelengths. Unfortunately, the WSS discussed therein is bulky, essentially including two WSS modules connected back-to-back. In addition, it is not readily expandable for a large number of ports. For example, at N=40 ports and K=80 wavelengths, it requires two arrays of 40×80 beam deflectors.
In U.S. patent application Ser. No. 12/367,160 filed Feb. 6, 2009 to Colbourne, which is hereby incorporated by reference, a M×N WSS module requiring significantly fewer beam steering elements (e.g., beam deflectors) is disclosed. Referring to FIG. 2, the M×N WSS 200 is shown having an input fiber array 202 of M input fibers, an input microlens array 204 of M microlenses, a collimating lens 206, a focusing lens 207, each lens having a focal length f, a diffraction grating 208 disposed one focal length f away from the lenses 206 and 207, a roof prism 210, a first micro-electromechanical (MEMS) micromirror array 212 disposed one focal length f away from the lens 207, a switching lens 214, a second MEMS micromirror array 216, and an output fiber array 218 of N output fibers.
In operation, a diverging light beam 221 emitted by a fiber 201 of the input fiber array 202 is collimated by a corresponding microlens of the microlens array 204 to form a spot 222 one focal length f away from the collimating lens 206. Even though the beam 222 is “collimated” at the spot 222, since the beam size is quite small, it continues to diverge, the divergence not illustrated, and is subsequently collimated by the collimating lens 206, which couples it to the diffraction grating 208. The diffraction grating 208 spreads the beam 222 into a plurality of sub-beams, each sub-beam carrying a separate wavelength channel (i.e., termed “wavelength channel sub-beams”). The plurality of wavelength channel sub-beams are dispersed by the diffraction grating in a plane parallel to the YZ plane in FIG. 2. The dispersed wavelength channel sub-beams are coupled by the focusing lens 207, through the roof prism 210 onto the MEMS micromirror array 212. The array 212 has M rows of K micromirrors, where K is the total number of wavelength channels, and is disposed so that each of the micromirrors is illuminated by a particular of the K wavelength channel sub-beams emitted by a particular of the M input fibers. The beam angle of each wavelength channel sub-beam reflected from a corresponding MEMS micromirror is determined by a tilt of the corresponding MEMS micromirror, in dependence upon a control signal, not shown, applied to each MEMS micromirror of the array 212. The switching lens 214 acts as an angle-to-offset converter. More specifically, since the beam angles of individual wavelength channel sub-beams are individually determined by the angle of tilt of corresponding micromirrors of the MEMS micromirror array 212, then the switching lens 214 will direct the wavelength channel sub-beams to fall on predetermined micromirrors of the second MEMS micromirror array 216. The second MEMS micromirror array 216 has N micromirrors, each micromirror being associated with a particular of N output fibers of the output fiber array 218. The role of the second MEMS micromirror array 216 is to couple a wavelength channel sub-beam falling onto its micromirror to the output fiber corresponding to said micromirror. Which wavelength channel sub-beam is coupled depends on the micromirror tilt angle that, in its turn, depends on a control signal, not shown, applied to the micromirror of the array 216. In this way, any one of the K wavelength channel sub-beams in the input fiber 201 is independently switchable into any particular one of the N output fibers, depending upon the individually controllable tilt angles of corresponding MEMS micromirrors of the arrays 212 an 216. Similarly, wavelength channel sub-beams 225 emitted by an input fiber 205 of the array 202 are independently switchable.
Notably, the M×N WSS module taught by Colbourne is particularly useful if only one signal needs to be sent to any one output port, such as when the output ports are directly coupled to receivers or transmitters. One advantage of this optical design is that only M rows of switching elements (instead of M+N rows) are required at the back end, while an N element array of switching elements is required at the front end (in front of the output ports). Accordingly, for an 8×20 WSS with 100 wavelength channels, a total of 820 switching elements (i.e., 8*100+20=820 switching elements) will be required. Compare this with the full cross-connect design taught in U.S. Pat. No. 6,711,316, which would require 28 rows of switching elements at the back end, or a total 2800 switching elements (i.e., 28*100=2800 switching elements).
Unfortunately, the use of the roof prism 210, which offsets light beams impinging onto the MEMS array 212 relative to light beams reflected therefrom so that the optical elements 202 and 204 can be disposed on the opposite side of the optical axis 240 relative to the optical elements 214, 216, and 218 to prevent mechanical interference, significantly increases the bulk of the M×N WSS module.